The normal physiological role of cholesterol or the biochemical nature of its involvement in certain pathological conditions of man and other species is not fully understood. A detailed study of the effects of dietary cholesterol in the guinea pig, a species highly sensitive to cholesterol-induced injury, promises to assist in the understanding of its normal function and its pathological effects in other species. We have found that the initial effects of exogenous cholesterol in the guinea pig is an expansion of the body cholesterol pool caused primarily by an inability to limit intestinal absorption of endogenous production, an increase of the ratio of unesterified cholesterol to esterified cholesterol in the liver and plasma, liver injury, the production of plasma lipoproteins abnormal in respect to their lipid composition and polypeptide constituents and striking changes in the morphology and composition of the red cells. This is followed several weeks later by a hemolytic anemia. We propose to study: 1) the cause for the species-specific accumulation of unesterified cholesterol by investigating the three major cholesterol-esterification systems - liver, plasma and intestinal mucosa; 2) the possible involvement of the immune system in the development of the hemolytic anemia by immunochemical methods; 3) the role of the abnormal lipoproteins in the net accumulation of cholesterol by the red cell membrane in vivo and in vitro; 4) the mechanism(s) by which the increased cholesterol content causes the morphological abnormalities of the red cell by the measurement of calcium concentration and fluxes and by attempts to map the localization of the excess cholesterol in the membrane; and 5) the formation and interconversions of lipoprotein density classes by the use as marker of the arginine-rich polypeptide with its associated antigenic determinant that characterizes the lipoprotein-apoproteins of cholesterol-fed guinea pigs and other species.